This invention relates to a safe cutting device for cutting a thin sheet or film such as a thin sheet or film of synthetic resin, aluminum foil, waxed paper, parchment paper, or glassine, to a method for cutting such thin sheet or film by use of the cutting device, and to a thin sheet or film dispenser provided with the cutting device in such a manner that the thin sheet or film stored within the dispenser may be drawn to a desired length from the dispenser and ripped along the edge of the cutting device.
The conventional cutting devices of general run such as represented by knives, scissors, and saws have sharp, large exposed blades. When they are touched accidentally by human flesh even very lightly, they tend to inflict serious wounds thereon. The class of cutting devices used for cutting thin sheets and films, the articles to which the present invention is directed, are no exceptions.
To permit convenient use of thin sheets and films, there prevails a popular practice of storing the thin sheets and films as rolled or folded in dispensers, drawing them to desired lengths out of the dispensers whenever they are desired to be used, and ripping them along the edges of cutting devices disposed at the outlets of the dispensers.
The cutting devices provided for the thin sheet or film dispensers are represented by metal plates containing saw-toothed cutting blades readily found in dispensers for synthetic resin films, aluminum foils, and waxed papers, etc. now enjoying widespread household use. A typical saw-toothed cutting blade used in a dispenser is illustrated in FIG. 23.
Unfortunately, the thin sheet or film dispensers which are provided with such metal-plate cutting devices incorporating saw-toothed cutting blades possess various defects such as are enumerated below.
Firstly, their users readily sustain injuries on their hands and fingers through inadvertent exposure thereof to a direct touch on the edges of such cutting blades.
Secondly, the blade of the tool used for forming saw-toothed cutting blades in metal plates to produce the cutting devices under discussion is susceptible to quick wearing by use. When the blade of the tool is worn even to a slight extent, burrs occur on the saw-toothed cutting blades of the cutting devices to be produced. These burrs often scratch the hands and fingers of the users of the cutting devices. Preclusion of the occurrence of these burrs necessitates frequent replacement of the cutting blade of the tool or setting of the saw-toothed edge of the tool. Consequently, the thin sheet or film dispensers provided with the saw-toothed cutting devices suffer from poor efficiency of production. In the attachment of the metal-plate cutting devices containing saw-toothed cutting blades to the cases of dispensers, the cutting edges of the devices must be positioned accurately at a fixed distance from the corners of the dispensers. If the cutting edges protrude excessively from the corners, they pose a danger at all times. If they protrude insufficiently from the corners, they fail to form desired cuts in the thin sheets or films. The great accuracy with which the cutting edges are required to protrude from the corners of the dispensers also constitutes itself one of the factors for the poor efficiency of production of the dispensers.
Thirdly, since most dispensers for thin sheets or films of synthetic resin, aluminum foil, or waxed paper are in the shape of elongated rectangular parallelepipeds as illustrated in FIG. 23, the aforementioned cutting devices fastened along one longitudinal corner of the dispensers with the cutting edges 609 protruding therefrom to a length of 1 to 2 mm are generally prevented from the possibility of the cutting edges 609 inflicting injuries on persons handling the dispensers at all the stages of distribution channel between the producers and the end users and, at the same time, protected against the possibility of their cutting edges 609 sustaining damage due to collision with hard objects. For this purpose, the front flaps 604 of the containers of dispensers 601 are extended past the upper surfaces 602 enough to form covering pieces 607 capable of keeping the cutting edges 609 covered thereunder at all times. Since the outermost portions of these covering pieces 607 are slanted because of the protrusion of the cutting edges 609 from the corners of the containers, they tend to prevent the dispensers from being neatly piled up in perfectly level positions as shown in FIG. 24 during storage, transportation, or display. When the dispensers piled up in heaps are suffered to tumble down, the impacts may be so serious as to possibly bend the cutting edges 609 out of their usefulness despite the protection offered by the covering pieces 607 or to cause separation of the covering pieces 607, with the result that the dispensers will have their market value degraded. Further, the protrusion of the leading ends of the covering pieces 607 from the rear surfaces 605 of the containers also interferes with neat stacking of the dispensers and causes separation of the covering pieces 607 themselves.
Fourthly, while the thin sheets or films such as of vinylidene chloride are not easily torn in the absence of a cut, they are readily torn from a very slight cut inserted in their boundaries. This attribute is not peculiar to vinylidene chloride films but is common to thin sheets or films of a variety of materials. Particularly in the case of rolled thin sheets or films such as those of vinylidene chloride which possess low tear propagation strength and ample self-tackiness and rolled thin sheets or films vested with tackiness, when the freshly cut ends of such thin sheets or films happen to adhere fast to the outermost walls of the rolls and the cut ends are subsequently desired to be separated from the rolls and drawn out, since the cut ends are zigzagged in the shape of a sawtooth because of the saw-toothed edges of the cutting devices used on the dispensers, the sheets or films tend to be torn from some of the V-shaped notches of the saw-toothed cut ends in the direction in which the sheets or films are being drawn out. Thus, once the cut ends are suffered to adhere fast to the rolls, it is extremely difficult for the adhering cut ends to be drawn out of the rolls afterward.
Various attempts have been being made with a view of overcoming the various defects mentioned above which are suffered by metal-plate cutting devices incorporating saw-toothed cutting blades. All these attempts nevertheless adhere to the basic concept of imparting cuts to the thin sheets or films by means of saw-toothed cutting edges. They are invariably directed to improvements in material, in blade strength, or in blade position relative to the dispensers. Thus, all the improvements brought about by the efforts heretofore made have not served to overcome the defects due to the saw-toothed edges of the cutting blades.
For example, since most dispensers for thin sheets or films are made of cardboard paper, it has been attempted to form parts of such cardboard dispenser cases in the shape of sawtooth and utilize the saw-toothed edges as cutting blades. Paper by nature is deficient in toughness. Even cardboard paper is easily bent under the shear stress exerted thereon when the thin sheet or film is ripped against the saw-toothed edge. Owing to the abrupt decrease of width in the individual points of the sawtooth coupled with the inferior binding force between the individual fibers making up the paper, the strength used in forcibly ripping the thin sheet or film against the sawtooth crushes the points of the sawtooth. Thus, the saw-toothed edges formed in the cardboard dispenser cases fail to withstand repeated use as cutting devices.
A method for enhancing the toughness of the points of the saw-toothed edges formed in the cardboard dispenser cases by causing the saw-toothed edges to be impregnated with an adhesive agent or resin solution has been disclosed by Japanese Patent Publication No. 36392/1973. This publication happens to contain a statement to the effect that a resin layer containing metal or glass powder is formed on the surfaces of the saw-toothed cutting edge. The cutting of a thin sheet or film as disclosed in this publication depends solely on the saw-toothed edge. The technical idea of the present invention which resides in effecting desired cutting of a thin sheet or film with a coarse file-like surface, a novel cutting device, is neither disclosed nor suggested anywhere in this patent publication.
A field test of this method has revealed that the method provides effective cutting for limited types of films, fails to cut the films smoothly, and leaves rugged edges in the cut ends. This is because the method effects the cutting of a thin sheet or film by causing the applied stress to concentrate at the sharp points of the sawtooth formed in the cardboard dispenser case. The rigid material such as glass powder used along the saw-toothed edge is solely intended to reinforce the saw-toothed edge of the cardboard paper. The greater part of the powdered rigid material is absorbed into the cardboard paper to form a base of impregnation therein and only a part thereof is used in the formation of a surface layer. Since the cardboard paper is a soft material, the extent to which the sharpness of the points of the saw-toothed edge is enhanced by use of the powdered rigid material is limited. This method cannot be expected to satisfy both safety and sharpness of the cutting edge of cardboard paper through the reinforcement contemplated thereby.
Because of these drawbacks suffered by such methods of improvement the dispensers for thin sheets or films which are provided with metal-plate cutting devices containing saw-toothed cutting edges and therefore entailing the various defects mentioned previously are still in popular use.